Visual tests to determine a subject's response to such variables as light intensity, color or stimulus size or to spatial or temporal patterns are common in ophthalmic examinations. One widely used visual test, known as perimetry testing, is designed to determine the range and sensitivity of a subject's visual field (Anderson, 1987; Johnson, 1983a). Many diseases of the eye, e.g., glaucoma or optic neuropathies, affect field of vision, and often these diseases are evidenced at an early stage, when treatment may be most effective. Additionally, the type and extent of many eye diseases, and the location of the disease along the visual pathway (i.e. the eye, the optic nerve, or the visual centers of the brain) can often be determined by visual- field examination.
In standard static perimetery testing methods, the visual field is partitioned into a number of discrete locations and the subject's threshold to visual stimuli at each of the locations is determined. Heretofore, this has been done by one of two methods. In a first approach, known as the accending methods of limits, the subject is presented with a series of stimuli of step-wise increasing intensity levels, until an "unseen" stimulus is first seen. This first seen stimulus intensity determines the visual threshold. In a second approach, known as the staircase method, the subject is presented with a series of stimuli of step- wise increasing intensity levels, until an "unseen" stimulus is first seen. Thereafter, the direction of the "staircase" is reversed, until a lower limit of detection is determined. The threshold is then refined and confirmed by two additional reversals of stimulus intensity. The staircase method involves at least 5 stimulus presentations at each location. Because of the additional reversals of stimulus intensity the staircase procedure is more accurate and reliable then the accending method of limits.
A more accurate method for determining intensity threshold is by a Modified Binary Search (MOBS), a method proposed by Tyrrell et al. (1988). This procedure has been used for measuring visual thresholds, but has not previously been applied to visual field testing. In this method, a selected-intensity stimulus, typically near the expected threshold level of the subject is presented, and depending on whether the stimulus is seen, that stimulus becomes the upper or lower threshold boundry. The intensity range is then divided up into a series of increasingly smaller half- intervals until the upper (seen) and lower (not seen) threshold boundries are within a defined range, e.g., 4 dB.
In prior-art perimetry methods, threshold values--either by the accending method of limits or the staircase procedure--are determined at each location in the visual field. The test method is therefore relatively time-consuming, in that many stimuli must be presented at each location. Typically, the time required for complete testing is between 15-30 minutes. Studies have shown a loss of reliability with increasing test time, which can be accounted for by fatigue factors (Johnson et al., 1988). This problem is aggravated in older patients, the group for whom perimetry testing is generally most important. In addition, the point-by-point approach lacks an efficient method of error correction, in that bad guesses or patient mistakes during the test can only be estimated by additional "catch" trials. Suspicious test values at any location can only be checked by a full retesting of the location after the field has been completely tested.